1. Field of the Invention
The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with a nuclear fuel rod grid with spring and dimple structures having chamfered edges for reduced pressure drop.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Representative grid designs include those of some of the patent applications cross-referenced above.
One popular conventional grid design, being illustrated and described in U.S. Pat. No. 4,492,844 to Kobuck et al and assigned to the assignee of the present invention, includes a multiplicity of interleaved inner and outer straps having an egg-crate configuration forming a multiplicity of cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and/or relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell project toward the center of the cell and frictionally engage or contact the respective fuel rod extending through the cell. Additionally, the outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid.
However, several drawbacks arise from this particular grid design. The vertical orientation and configuration of the fuel rod engaging springs require that the springs and thus the grid be of substantial height in order for the springs to have the desired amount of resiliency to function properly. This requirement increases the amount of parasitic structural material utilized in the fuel assembly and the pressure drop through the fuel assembly. Also, the projection of the springs and dimples into the grid cells and thus across the upward path of coolant flow along the fuel rods and through the grid cells further increases the pressure drop through the fuel assembly.
A recent U.S. Pat. No. to King et al (4,756,878) has proposed an approach to reducing pressure drop of cooling liquid through a grid by convexly contouring the upstream (usually the lower) edges of the grid straps and by tapering the downstream (usually the upper) strap edges. In particular, the upstream grid strap edges are made streamlined to semicylindrical or semiellipsoidal by any of various techniques, such as beveling (coining) and etching, applying abrasive material, or electron or laser beam melting.
While the approach of the above-cited King et al patent may be a step in the right direction, it falls short of providing an optimum solution to the problem of reducing pressure drop caused by fuel rod spacer grids in a nuclear fuel assembly It is perceived by the inventor herein that a need still exists for an improved grid structure which will avoid the above drawbacks without presenting new ones in their place.